FBSD: adhere to gop_lookupname() semantics

The current semantics are that gop_lookupname() returns an unlocked
vnode; the previous code was written to a different semantic that
a locked vnode should be returned.

This makes a disk cache more likely to work on FreeBSD, but such
configurations remain not very tested.

Reviewed-on: http://gerrit.openafs.org/11317
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit 774d77e056515ae3e87c8f0be8e133c3cdb36bbb)

Change-Id: I4ae9c269412c2ce6b9ec39a841ea87f27630591e
Reviewed-on: http://gerrit.openafs.org/11448
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>